1. Field of the Invention.
The present invention relates to devices for measuring thread characteristics and, specifically, to a device for measuring the thread minor diameter.
2. Description of the Prior Art.
To meet todays demand for assembly efficiency and product reliability, American industry is requiring consistently higher quality components. Accurate thread gaging is one way to improve fastener quality to meet the needs of industry.
There are two basic types of thread gaging. The older, but still widely used type, is the "fixed limit" or "attribute" gaging. External threads are measured with ring gages and internal threads with plug gages. This type gaging only establishes the outer boundaries of the thread and simply indicates whether or not a particular thread falls inside or outside of these boundaries. This type gaging was generally developed in the 1920s and is becoming inadequate for today's higher quality and consistency demands.
Variable or indicating thread gages comprise the second type of commonly available gaging for threads. These devices were developed with dial indicating systems in the 1940s and were improved in the 1980s to include the use of electronic readouts. Variable thread gages provide actual measurements of the thread characteristic being inspected. The threads acceptability is determined by comparing those measurements to the applicable thread standard. The variable gage is generally faster to use and provides greater accuracy than the fixed limit gage.
A variety of variable gages are known for measuring such thread characteristics as pitch diameter, minor or root diameter and major or outer diameter of a screw thread. In the case of root diameter measurements, the prior art variable gages have typically employed a pair of oppositely arranged single point locators, often in the form of V-shaped blades. Such an arrangement allowed the screw being measured to tilt or cant on the test fixture, thereby affecting the accuracy of the root diameter measurement.
The present invention has as its object to provide a measuring device for measuring the minor diameter of a screw thread quickly and accurately.
The invention also has as its object to provide a minor diameter gage having lower gage blades which are independently moveable to a correct gaging position to eliminate gaging problems associated with the devices of the prior art.
Another object of the invention is to improve the accuracy of existing root diameter thread gages without adding significantly to the cost or complexity of the gage.
Additional objects, features and advantages will be apparent in the written description which follows.